The present invention relates to valve control systems for internal combustion engines of the type in which the movement of an engine poppet valve is controlled in response to rotation of a cam shaft, and more particularly, to such a valve control system in which the cam shaft has a cam profile including both a high lift portion and a low lift portion.
Even more specifically, the present invention relates to such a valve control system including a dual lift rocker arm assembly of the type having both a high lift cam follower and a low lift cam follower (for engagement with the high lift portion and the low lift portion, respectively, of the cam profile). Although the terms “high lift” and “low lift” can have various meanings when used in regard to valve control systems for engine poppet valves, it should be understood that, within the scope of the present invention, all that is required is that one cam profile provide a relatively higher lift of the engine poppet valve while the other cam profile provides a relatively lower lift of the engine poppet valve. Within the scope of the invention, the “low lift” could actually comprise zero lift, or could comprise some finite lift amount which is greater than zero lift, but somewhat (or substantially) less than the “high lift”.
In a typical dual lift rocker arm assembly, of the type which is now well known in the art, there is provided an outer rocker arm and an inner rocker arm, with those two rocker arms typically being pivotally connected relative to each other toward one axial end thereof. In addition, the typical, prior art dual lift rocker arm assembly includes some sort of latch mechanism, operable to latch the inner rocker arm to the outer rocker arm, such that the two rocker arms move in unison about a fulcrum location, such as the ball plunger of a hydraulic lash adjuster. This “latched” condition, as described above, would typically, but not necessarily, correspond to the high lift mode of operation of the valve control system. When the latch mechanism is in the “unlatched” condition, the inner and outer rocker arm are free to pivot relative to each other, and this unlatched condition would typically, but not necessarily, correspond to the low lift mode of operation of the valve control system.
Dual lift, latchable rocker arm assemblies are illustrated and described in U.S. Pat. No. 5,524,580; 5,584,267; and 5,697,333, all of which are assigned to the assignee of the present invention, and incorporated herein by reference.
In the dual lift rocker arm assemblies of the above-incorporated patents, there is provided some sort of electromagnetic actuator for controlling the operation of the latching mechanism. Although such electromagnetic actuation of the latching mechanism has been found to operate in a generally satisfactory manner, the resulting need for a separate electromagnetic actuator for each rocker arm assembly would add substantially to the cost of the overall valve control system, and in many applications, would require much more space for “packaging” than is available in the typical engine cylinder head.
Those skilled in the art have attempted to provide a means of actuation for the latching mechanism of a dual lift rocker arm assembly, which would overcome the prior art problems discussed above, by utilizing hydraulic pressure. Specifically, those skilled in the art have attempted to utilize, to control the latching mechanism, a variable hydraulic pressure within the plunger of the hydraulic lash adjuster, which serves as the fulcrum location for the rocker arm assembly. Such an actuation arrangement is illustrated and described in U.S. Pat. Nos. 5,544,626 and 6,668,779, both of which are incorporated herein by reference.
Although the rocker arm assemblies of the above-incorporated patents, in the immediately preceding paragraph, do provide at least the potential for substantially improved actuation of the latching mechanism, the need to communicate the low pressure (control) fluid from the lash adjuster to the latching mechanism has somewhat complicated the design of the rocker arm assembly. This is especially true when it is recognized that there are various other design criteria for rocker arm assemblies which must be observed, in order to achieve the best possible overall performance of the valve control system. For example, in order to improve the dynamic behavior of the valve control system, it is desirable to reduce the inertia of the rocker arm assembly. One way of reducing the inertia is to locate as much of the mass of the rocker arm assembly as close as possible to the fulcrum location. Therefore, it is recognized that it is desirable to have the pivot axis, between the inner and outer rocker arms, disposed adjacent the fulcrum location, such that the torsion spring, which biases the rocker arms relative to each other, may also be near the fulcrum location.
Unfortunately, in the dual rocker arm assembly of the above-incorporated U.S. Pat. No. 6,668,779, in order to utilize a control fluid from the hydraulic lash adjuster to control the latching mechanism, it was necessary to add a piston member (the only function of which was to move in response to changes in control pressure), with the movement of the piston member being transmitted from the piston member to the latching mechanism at the opposite end of the rocker arm assembly by means of a separate slider element, having no function other than to move the latching mechanism in response to movement of the piston member The added cost and complexity of the arrangement in the rocker arm assembly of the '779 patent, as well as the added mass and inertia of the assembly, make the overall assembly less than desirable commercially.